While the present discussion is directed towards apparatus for heating blood or plasma, it is to be understood that the present invention may find application in other areas involving a range of geometries, capacities and subject liquids.
Blood and blood products are generally refrigerated for the purposes of storage at approximately 1-6 degrees celcius. Consequently, infusion of such fluids at below body temperature may result in shock, hypothermia or cardiac dysfunction. Additionally, such conditions can be aggravated by the infusion of physiologically cold fluids. Accordingly, it is known, indeed required, in the art to heat such fluids prior to infusion into a patient.
The minimum acceptable infusion temperature will depend on the condition of the patient, the duration of the infusion, the volume of liquid to be administered to the patient, and the patient's blood volume prior to infusion. However, generally the infusion temperature must be at least at or near the patient's body temperature.
In addition to the temperature criteria discussed above, it is known that the combination of insufficiently heated blood with high infusion rates can result in destabilisation of the patient's thermoregulatory system. Alternatively, excessive warming may damage the red blood cells.
Accordingly, it is vital that the infusion temperature be closely monitored and controlled in response to a particular patients physiological condition and the other factors mentioned above.
To the present time it is known to warm blood, plasma or other medical fluids, using water bath warming, circulating fluid and dry heat devices. Water bath blood warmers incorporate a warm water reservoir set to maintain a constant temperature of between approximately 36 and 40 deg C, a bag, or coil of tubing is immersed in the water bath. The blood or plasma is then warmed by passing it through the bag or coil prior to infusion. A variation on this is the counter flow circulating fluid device, where two concentric tubes form a heat exchanger, the blood or plasma to be heated is passed through the inner tube, while the heated fluid from the reservoir (usually water) is pumped in the opposite direction through the outer tube. Dry heat warmers warm the blood by passing it through tubing or a bag which is located between heating plates or by passing it through a disposable cuff style bag which is wrapped around a cylindrical heating element.
Many blood warming systems known in the art are significantly limited in that high infusion rates cannot be sustained in combination with sufficient blood heating. A further difficulty with prior art blood warming devices, particularly heated water bath units, is that the blood may become contaminated by contact with the heated liquid. It is of prime importance that the blood flowing through the blood warmer be contained within a sterile environment. Reported cases of blood contamination in the context of water bath blood heaters, indicates that this type of blood warmer is particularly susceptible to such contamination effects. While repeated and thorough cleaning of the water bath may avoid contamination, such processes can be time consuming and necessitate the dismantling of the blood warming device.
Another significant limitation of prior art blood warmers is that they generally, because of their construction, do not lend themselves to mobility and ease of use. Particularly in the context of field operation or warfare environments, where conventional blood heaters may be difficult to operate properly.
As noted above, the need for potentially high flow rates coupled with the requirement that the blood temperature be elevated and regulated precisely, means that conventional blood warming systems exhibit significant limitations in function and application. Accordingly, there exists a need for a blood warming unit which, amongst other things, is compact, portable, resistant to contamination and, most importantly, provides high flow rates in combination with precisely controlled heating.
A type of fluid warming device which represents a major departure from those known in the art is that which exploits inductive heating. Such devices are discussed in U.S. Pat. No. 5,319,170 (Cassidy) and PCT/GB89/00629 (Curran). Both of the devices described in these specifications incorporate a conductive heating element forming a shorted secondary winding of a transformer, which is magnetically coupled to a primary circuit powered by alternating current. The inductive coupling produces currents in the secondary thereby generating heat which is transmitted to the fluid in contact with the secondary. Such devices are advantageous in that they are electronically operated and are thus particularly useful for remote use. Use in remote locations does not lend itself to the application of relatively cumbersome water bath or similar blood heating units.
While the device described in Cassidy does, to some degree, overcome some of the above mentioned disadvantages, it is considered that blood travelling along different paths through the device will be subjected to different heating times, thereby raising the possibility of some of the blood being heated beyond its maximum permissible level. This could either result from paths being of different lengths, depending on their radius from the centre of the toroid, or from stagnation occurring such as in areas either side of the inlet port.
It is also believed that the relatively loosely coupled magnetic circuit used in the Cassidy device may result in unwanted electromagnetic emissions. Such emissions may interfere with monitoring equipment used in, for example, an operating theatre environment as well as electronic components in the patients immediate environment. It is also desirable to reduce the patient's exposure to unwanted electromagnetic fields. While the effect of such electromagnetic fields is still uncertain at this time, it is prudent to construct such a device so as to reduce unwanted electromagnetic emissions as effectively as possible.
The Curran device discloses an induction heater incorporating a mesh conductive heating element in the form of a spiral. The inner edge of the spiral is attached to the outer edge of the spiral by means of a shorting strap thereby forming a shorted secondary winding. However, the Curran device is constructionally complex in that the spiral wound heating element is formed from mesh and must be supported at either end by some suitable means and must also be shorted to render the secondary closed. Further, while the mesh structure of the heating element disrupts the axial flow of the fluid thereby causing transverse turbulence which may result in more homogeneous heating, it is likely that such turbulent flow may significantly reduce the flow rate through the device. Further, being coreless the Curran device will have a relatively loosely coupled magnetic circuit. In situations such as this, where the field is less constrained, to increase the magnetic flux density a greater number of turns on the primary are required. This will result in a bulkier, more expensive and potentially less efficient unit.
Further, it is believed that the Curran device will produce more electromagnetic noise than a central core device having a more tightly constrained magnetic circuit.
Accordingly, it is an object of the present invention to provide an inductive fluid warmer which is compact, light and portable, of simple construction and with the heat exchanger chamber consisting of a cheap disposable cartridge that is not susceptible to contamination by a thermally coupled heating means, poses a minimal or reduced risk of electromagnetic interference or at least mitigates some to the above mentioned disadvantages and it provides the public with a useful choice.